This invention relates to flowmeters and more particularly, but not exclusively, to flowmeters for the provision of blood velocity measurements by use of non-invasive ultrasonic Doppler techniques.
A difficulty in obtaining reproducible measurements with such techniques is that the factor of proportionality between observed Doppler shifts and velocity involves the cosine of the angle between the direction of the ultrasonic beam and the direction of flow and accurate knowledge of this angle is required except when it is possible to aim the beam so that the relevant angle is close to zero. Since the great majority of the more readily accessible blood vessels such as the limb arteries and the carotids run more or less parallel to the body surface, it is not possible to render the beam/vessel angle close to zero and it is therefore necessary to establish this angle with some accuracy if absolute velocity measurements are required, or to ensure that the angle remains constant if indications of velocity changes are desired by serial observations. In either of these last situations practical difficulty arises. The present invention has been arrived at from a consideration of these difficulties and may enable simplification of measurements of flow velocity.